1. Field of the Invention
The present invention relates to the measurement of the two-phase flow of vapor and liquid from the steam generation section of a large utility boiler to manifest the liquid phase, which is recirculated within the boiler. More specifically, the invention relates to utilizing the pressure drops, or the pressure differentials, within and across the vapor-liquid separator to manifest the amount of vapor and of liquid in the two-phase mixture and form a guide to design, which, when applied, will avoid excessive liquid carryover with the vapor to the downstream turbine utilizing the vapor.
2. Prior Art
In the large utility boiler, be it thermally energized by the combustion of fossil fuel, or by nuclear reaction, there is ultimately a flow of two-phase vapor and liquid into the upper vapor drum. The vapor from this drum flows to a motive device, such as a turbine to ultimately perform work, usually the generation of electricity. In the vapor drum, it has been customary to mount a plurality of vapor-liquid separators utilizing centrifugal force. These separators receive the vapor-liquid mixture from a plenum and reduce the liquid content of the vapor to 1/2% or less, by weight, of the total flow.
It is not unusual to think of the overall production of these utility boilers, as in the order of 6 to 8 million pounds of vapor per hour. More closely focusing on the vapor-liquid separator mounted in the vapor drum, it enlarges the perspective to be aware that each separator will receive 30 to 50 thousand pounds per hour of vapor and 11/2 to 41/2 times that amount of liquid. As said before, the separator is designed to reduce the liquid content of the vapor output to 1/2% or less.
This mixture flows to the vapor separator at a rate in the order of 20 feet per second, which is increased in circumferential flow, to the order of 40 feet per second to generate centrifugal force utilized to separate the two phases.
Another physical dimension for purposes of perspective is the visualizing of the size of the separators, as in the order of 10" in diameter and 40" high.
Behind the foregoing physical parameter of quantity and size is the fact that measurement of the internally recirculated liquid component of the mixture has not been heretofore directly measured. The liquid has been spun from its association with vapor and discharged from each of the separators, having been joined without a practical way to measure any one, or all, of the liquid flows directly. Of course, this measurement has been indirectly calculated, but a more accurate measurement of the flows for specific separators is needed to enable more efficient operation of the system, and to prevent excessive carryover of liquid with the vapor. Excessive liquid in the vapor supplied to the turbine results in early formation of droplets of liquid that erode the turbine blades, in addition to reducing the vapor cycle efficiency. To insure that good quality vapor is supplied to the turbine, extreme conservatism is required in the pressure drops across the separator, and in the number of units used. During periods of low power operation, the vapor flow is a fraction of the full power flow rate, and the vapor velocity is so low that there can be gravity separation of the liquid. If this occurs, there is little or no recirculation of the liquid in the boiling region of the tubes. The duration of this undesirable operating condition can be minimized by system controls, which can be tuned to the measured recirculating liquid flow.
Strictly for the purposes of giving a restrictive scope to terminology, it is to be understood that the more common, practical, vapor is steam generated by heating water. When a vapor generator is referred to, it is to be understood that this vapor is most commonly, simply steam. Further, the liquid separated from the steam is water, and violence will not be done to the scope of the invention by consistent use of this terminology. There remains the problem of measuring the flow rate of water associated with steam.